1. Technical Field
This document relates to a method, system, and apparatuses for simultaneous heat and mass transfer utilizing a carrier-gas at various absolute pressures.
2. Background Art
In view of the increasing need to obtain clean water or to minimize the volume of waste waters, or both, many conventional separation techniques have been studied and developed. One example of such a conventional separation technique is desalination. Many technologies have been used to perform desalination, but economic factors have dictated the preferred technologies. For example, reverse osmosis (RO) is good for desalination of mild brackish water (e.g., less than 1000 ppm total dissolved solids (TDS)). This preference results from the fact that other technologies utilize techniques which require costly phase changes of the liquid, such as boiling a liquid into a gas. In contrast, RO employs low-pressure pumps (less than 100 psi) to force water through semi-permeable membranes, thereby consuming less energy than a boiling process.
However, RO is ineffective in purifying water containing non-filterable suspended particulates. For example, the process of chemical mechanical polishing (CMP) used by the silicon industry discharges an aqueous slurry containing about 15% by weight of aluminum oxide particles. Being less than about 1 micron in size, these particles are non-filterable and readily foul RO membranes. As a result, the CMP slurries are typically discharged to the environment, which has been historically less costly then investing in low capacity thermal technologies. Still another example is the purification of river water. Many rivers, such as the Colorado River, contain silt in the 1 micron range which can foul RO membranes, thereby increasing the maintenance and/or pretreatment costs of RO operations.
For the more TDS intense aqueous applications, such as waste water streams and sea water, other mechanical and thermal technologies economically compete with RO. In sea water desalination, for example, the RO pump pressures increase to 1000 psi. Furthermore, feed waters require expensive pretreatments in order to protect and extend the life of the membranes. Technologies in competition with RO for seawater desalination include mechanical vapor compression (MVC), multi-stage flash distillation (MSF), and multi-effect distillation (ME) with and without thermal vapor compression. MVC technology requires shaft power to drive its compressor. The motor can be either electrically or thermally driven. Unfortunately, electrically driven MVC plants consume more electricity than RO units for sea water desalination. The thermally driven processes (MSF and ME) use heat to provide a temperature-driving force at different stages of boiling and condensing and at various stages of pressures. Thus, the thermally driven plants attempt to reuse the high temperature from the applied heat as many times as is economically possible in order to minimize operating costs. This energy reuse factor economically varies from 6 to 12. The energy reuse factor is also referred to as the gain output ration (GOR). As the GOR increases so does the equipment capital cost. The optimum GOR value depends upon typical cost variables, such as plant capacity, cost of energy, cost of materials, etc.
One attempt to overcome these problems involved the use of simultaneous heat and mass transfer in stages utilizing a carrier-gas. In such a staged apparatus, the liquid is sprayed onto each side of a heat transfer wall to obtain enhanced heat and mass transfer coefficients and reduce the required surface area of the heat transfer wall. This technique allowed the film gas heat transfer coefficients to be in the range of about 100 to 300 WIm2 CC, which resulted in condensate production fluxes in the range of about 2.3 to about 6.8 kg of condensate per hour/in2 of heat transfer wall. Since spraying the liquid is essential to maintaining the enhanced heat transfer coefficients, pumps were needed to force the liquid through spray nozzles. However, a staged configuration was required to limit the mixing by the pumps of the liquid compositions and temperatures at different locations of the apparatus. As a result, each stage required a dedicated pump and nozzle, and about 50 to 100 stages were required to achieve GOR values of about 10 to 20. Since each side of the heat transfer wall had to be wetted, this meant that the total number of pumps and nozzles required was from about 100 to about 200. Accordingly, the increased complexity of these apparatus was detrimental to economic and reliable operation.